1. Field of the Invention
The present invention relates to an information processing apparatus required to efficiently search for regions corresponding to a region of interest between images having different imaging conditions such as modality (e.g., ultrasound, MRI, etc.), imaging body posture, imaging date and time, and a control method thereof.
2. Description of the Related Art
In the medical field, a doctor diagnoses a lesion area by displaying, on a monitor, a medical image (three-dimensional image data including a tomographic image group which represents three-dimensional information inside an object) obtained by capturing images of the object, and interpreting the displayed medical image. As medical image collection apparatuses used to capture the medical image (referred to as “modality” herein), an ultrasonic image diagnosis apparatus, magnetic resonance imaging apparatus (“MRI”), X-ray computed tomography apparatus (“X-ray CT apparatus”), and the like are known.
It is difficult to correctly diagnose the state of a lesion area by merely observing medical images captured by these individual modalities. Hence, attempts have been made to correctly diagnose the state of a lesion area by comparing lesion areas in medical images captured using different modalities or captured at different dates and times.
In order to use a plurality of types of medical images in diagnosis, it is important to identify (associate) lesion areas and the like in the respective medical images. Since it is difficult to attain automatic identification by means of image processing due to the influence of using different modalities, deformations of the object, and the like, it is common practice for an operator such as a doctor to manually identify lesion areas while observing the images. The operator observes an image of a lesion area of interest pointed out in one medical image (referred to as a “reference image” hereinafter) to examine similarities of the shape of the lesion area, the appearance of its surrounding portion, and the like. Then, the operator searches for and identifies a lesion area (“corresponding lesion area”) corresponding to that lesion area from another medical image (to be referred to as a “target image” hereinafter) based on these pieces of information. In this case, if an apparatus which presents a medical image has a function of estimating and presenting the position of the corresponding lesion area on the coordinate system of the target image based on that of the lesion area of interest on the coordinate system of the reference image, the operator can search for the corresponding lesion area based on that estimated position.
Thus, an attempt has been made to calculate a relationship between the coordinate systems of an ultrasonic tomographic image as a target image and a reference image by measuring the position and orientation of an ultrasonic probe and to estimate the position of the corresponding lesion area on the coordinate system (ultrasonic coordinate system) of the ultrasonic tomographic image, thereby guiding an operation of the probe. For example, Japanese Patent Laid-Open No. 2008-246264 discloses a technique which calculates a distance and direction to the center of a target (lesion area of interest) set on a reference image (a three-dimensional tomographic image obtained using an MRI apparatus or the like) with respect to the current ultrasonic tomographic image, and displays an three-dimensional arrow image and numerical values based on the distance and direction. Then, since the user can visually recognize the distance from the current ultrasonic tomographic image to the target, he or she can easily recognize the correspondence (positional relationship) between the reference image and the ultrasonic tomographic image.
Also, Japanese Patent Laid-Open No. 2008-212680 discloses a technique which superimposes, when image tracking points (of a lesion area of interest) selected from a previous ultrasonic tomographic image are given, a square having a size and color based on distance and direction from the current ultrasonic tomographic image on the current ultrasonic tomographic image as an in-plane pointer. Thus, upon counting the number of nodules in a thyroid, the number of metastases in a liver, and the like, even when the angle and position of a probe are changed, the user can discriminate whether the currently visualized structure is one that is being newly identified or has already been identified and counted.
The measurement precision with which the position and orientation of an ultrasonic probe are measured is not perfect, and the shapes of an object at the capturing timings of the reference image and ultrasonic tomographic image do not always match. For this reason, the estimated position of the corresponding lesion area on the coordinate system of the ultrasonic tomographic image includes errors, and has a positional shift from that of the actual corresponding lesion area. However, the display processes disclosed in Japanese Patent Laid-Opens Nos. 2008-246264 and 2008-212680 do not consider any positional shift. For this reason, the user may not identify (find) the corresponding lesion area, depending on a degree of the positional shift. In this case, the user consequently searches the entire ultrasonic tomographic image for the corresponding lesion area, resulting in poor search efficiency.
Hence, the present inventors have proposed a technique for solving this problem, in Japanese Patent Application No. 2011-084495. According to this literature, a range (existence range) where the corresponding lesion area may be on the ultrasonic tomographic image is calculated in consideration of position estimation errors. Then, the existence range is presented on the ultrasonic tomographic image as a guide used when the user searches for the corresponding lesion area. Then, since the user can recognize a range (search range) where the corresponding lesion area is to be searched for, he or she can efficiently search for and identify the corresponding lesion area.
On the other hand, as a method of improving search efficiency by allowing the user to refer to the positions of the corresponding lesion area to be searched for and the ultrasonic tomographic image in an overview manner, information of a lesion area in a medical image and position information of a probe are displayed on a body mark which represents an outline shape of a target object. For example, Japanese Patent Laid-Open No. 2008-086742 discloses a method of calculating a transformation rule required to attain coordinate transformation from a position on a breast of a patient at the time of ultrasonography onto a standard body mark which represents a breast, and displaying a position of an ultrasonic probe on the body mark (by normalizing that position to that on the body mark coordinate system). Also, Japanese Patent Laid-Open No. 2008-279272 discloses a method of generating a 3D body mark by volume rendering of a target object captured by an X-ray CT apparatus or MRI apparatus, and displaying graphics indicating positions of a lesion area and probe on the 3D body mark to overlap each other. This method can support a probe operation when a doctor searches for a lesion area at the time of ultrasonography.
However, since Japanese Patent Application No. 2011-084495 merely displays one section of the three-dimensional existence range of the corresponding lesion area, it is not easy to recognize which location of the three-dimensional existence range is extracted. For example, when the three-dimensional existence range is given as a sphere, a two-dimensional existence range on the tomographic image is always displayed as a circle independently of which crossing portion between the ultrasonic tomographic image and three-dimensional existence range is the one being displayed. Therefore, it is not easy for the user who does not know the size of the sphere to recognize whether the current cross position is near the center of the sphere or the end of the sphere. When the ultrasonic tomographic image does not cross the existence range, it is not easy to recognize how far the ultrasonic probe must be moved (“moving distance” of the probe) to reach the existence range.
On the other hand, in Japanese Patent Laid-Open No. 2008-086742, since the position of the lesion area on the 2D body mark cannot be recognized, it is difficult to improve search efficiency. Also, in Japanese Patent Laid-Open No. 2008-279272, since the doctor has to recognize the positional relationship between the ultrasonic tomographic image and lesion area on the 3D body mark, which may be unfamiliar and unlike the familiar 2D body mark, the search efficiency may often drop. Furthermore, even when the ultrasonic tomographic image and the position of the lesion area are displayed on the 2D body mark based on Japanese Patent Laid-Open Nos. 2008-086742 and 2008-279272, a three-dimensional positional relationship between the ultrasonic tomographic image and corresponding lesion area as well as the depth direction of the body mark cannot be recognized. For this reason, an accurate moving distance of the ultrasonic probe required to capture the corresponding lesion area cannot be detected.
As described above, according to the above-described documents, the positional relationship between the corresponding lesion area or a region defined based on the corresponding lesion area and the ultrasonic tomographic image cannot be easily recognized.